1. Field of the Invention
The present invention relates generally to a method for preparing very high molecular weight polycarborane siloxanes. More specifically, the present invention relates to such a method in which the molecular weight of the polycarborane siloxanes is significantly and reliably increased by providing each of the reactant materials in an ultra-pure state by successive recrystallization procedures.
2. Description of Related Art
Materials which are used in a space environment must be able to withstand extreme conditions, such as exposure to temperatures from -100.degree. to +250.degree. F. (-73.degree. to +121.degree. C.) for extended periods of time, such as 10 years, and exposure to vacuum conditions in space. In addition, materials in space are exposed to high energy ultraviolet radiation, high energy protons and high energy electrons, all of which can produce significant damage.
In the case of solar cells used in space, a cover glass, usually formed of silica, is bonded to the front of each cell to protect the photovoltaic junction from radiation and particle damage. However, even with such a glass cover, high energy electron damage does occur. It has been found that the effect of electron radiation can be minimized if the cells are run at higher temperatures or periodically cycled to higher temperatures. In the case of silicon solar cells, electron damage can be annealed and the cell efficiency optimized by heating the cells to about 60.degree.-100.degree. C. At these temperatures, known adhesives used to bond the cover glass to the solar cell, such as dimethyl silicone resins, are sufficient. However, gallium arsenide type solar cells are currently being developed which have much higher efficiency than silicon solar cells at increased temperatures. In order to anneal the gallium arsenide cells for electron damage, a temperature in the range of 250.degree.-350.degree. C. is required. Known cover glass adhesives, such as DC-93-500, a two-part room temperature vulcanizing dimethyl silicone resin available from Dow Corning, have been indicated to be operational at temperatures up to 200.degree. C. Thus, a need exists for an adhesive which not only meets the previously discussed requirements of the space environment, but also can withstand annealing temperatures in excess of 200.degree. C.
One material which has been previously studied for its thermal stability is a class of ultra-high molecular weight carborane siloxane polymers, as described by Stewart et al in the publication "D2-m-Carborane Siloxanes. 7. Synthesis and Properties of Ultra-High Molecular Weight Polymer," in Marcomolecules, Vol. 12, No. 3, May-June 1979, at pages 373-377. In the method of Stewart et al, the carborane siloxane polymers are formed by:
(a) forming a slurry of carborane bisdimethyl silanol in dried chlorobenzene solvent and cooling the slurry to -10.degree..+-.5.degree. C.; PA0 (b) adding to the slurry a mixture of dimethylbisureido silane and methylphenylbisureido silane to form a reaction mixture at -10.degree..+-.5.degree. C.; PA0 (c) separating from the reaction mixture a silanol end-capped prepolymer of the polycarborane siloxane polymer; PA0 (d) dissolving the prepolymer in chlorobenzene to form a solution; and PA0 (e) adding to the prepolymer solution an excess of the above-noted bisureido silanes. PA0 (i) providing carborane bisdimethyl silanol in an ultra-pure state by dissolving said carborane bisdimethyl silanol in a dry, oxygen-free mixture of hexane and toluene by heating to approximately 70.degree. C. in an inert atmosphere to form a first solution, filtering said first solution, and cooling said first filtered solution to form a first crystalline product, and repeating said dissolving of said first crystalline product, said filtering, and said cooling for a total of 2 to 5 additional times; PA0 (ii) providing dimethylbisureido silane in an ultra-pure state by dissolving said dimethylbisureido silane in a dry, oxygen-free mixture of diisopropyl ether and tetrahydrofuran by heating to approximately 60.degree. C. in an inert atmosphere to form a second solution, filtering said second solution, and cooling said second filtered solution to form a second crystalline product, and repeating said dissolving of said second crystalline product, said filtering, and said cooling one time; and PA0 (iii) providing methylphenylbisureido silane in an ultra-pure state by dissolving said methylphenylbisureido silane in dry, oxygen-free diisopropyl ether to form a third solution, filtering said third solution, adding dry hexane to said third filtered solution to form a mixture and cooling said mixture to room temperature, to form a third crystalline product and repeating said dissolving of said third crystalline product, said filtering, said adding of hexane and said cooling two additional times; PA0 (a) providing said ultra-pure carborane bisdimethyl silanol in dried chlorobenzene solvent, to form a slurry and cooling said slurry to -15.degree..+-.5.degree. C.; PA0 (b) adding to said slurry a mixture of said ultra-pure dimethylbisureido silane and said ultra-pure methylphenylbisureido silane to form a reaction mixture at -10.degree..+-.5.degree. C.; PA0 (c) separating from said reaction mixture a silanol end-capped prepolymer of said polycarborane siloxane polymer; PA0 (d) dissolving said prepolymer in chlorobenzene to form a solution; and PA0 (e) adding to said prepolymer solution an excess of said ultra-pure bisureido silane selected from the group consisting of dimethylbisureido silane, methylphenylbisureido silane or a mixture thereof, to form said very high molecular weight polycarborane siloxane polymer.
Stewart et al reported that the polymers so formed had molecular weights in excess of 10.sup.6, which was believed to be due to the technique of the reverse addition of the bisureido silanes to the carborane disilanol in chlorobenzene. However, as disclosed by Stewart et al at page 375, right column, first and second full paragraphs, consistent and reliable results were not achieved. One problem was that a reliable technique for purifying the prepolymer was not found, and consequently the prepolymer was degraded by reaction with amine by-products. Another problem was that many of the prepolymer samples were not capable of being advanced to the high molecular weight polymers, and no cause for this difficulty was defined. In addition, attempts to replicate the experiments of Stewart et al did not result in polymers of the highest molecular weight reported by Stewart et al, as discussed in further detail herein in Example 6. As discussed by Stewart et al, optimum mechanical and thermal properties of these polymers occur only at high molecular weights.
Since the carborane siloxane polymers of Stewart et al at high molecular weight have desirable high temperature properties and could be useful in a space environment, it would be advantageous to have a process for synthesizing these polymers reliably at very high molecular weights.